Personal Digital Assistants (PDAs) have become more and more common in today's society. The term PDA refers generally to mobile computer systems, typically handheld, which users employ for a variety of tasks such as storing telephone and address lists (databases), calendaring information, task (i.e., to-do) lists, etc. Some PDAs also incorporate a wireless communication link, allowing the unit to operate as a portable facsimile device, Internet access device and/or pager. Further, PDAs can be configured to operate with Global Positioning System (GPS) receivers as described in U.S. Pat. No. 5,528,248 to Steiner et al., entitled "Personal Digital Location Assistant Including a Memory Cartridge, A GPS Smart Antenna and a Personal Computing Device" assigned to the assignee of the present invention and incorporated by reference herein.
The GPS utilizes signals transmitted by a number of in-view satellites to determine the location of a GPS antenna which is connected to a receiver. Each GPS satellite transmits two coded L-band carrier signals which enable some compensation for propagation delays through the ionosphere. Each GPS receiver contains an almanac of data describing the satellite orbits and uses ephemeris corrections transmitted by the satellites themselves. Satellite to antenna distances may be deduced from time code or carrier phase differences determined by comparing the received signals with locally generated receiver signals. These distances are then used to determine antenna position. Only those satellites which are sufficiently above the horizon can contribute to a position measurement, the accuracy of which depends on various factors including the geometrical arrangement of the satellites at the time when the distances are determined.
Distances measured from an antenna to four or more satellites enable the antenna position to be calculated with reference to the global ellipsoid WGS-84. Local northing, easting and elevation coordinates can then be determined by applying appropriate datum transformation and map projection. By using carrier phase differences in any one of several known techniques, the antenna coordinates can be determined to an accuracy on the order of .+-.1 cm.
Although U.S. Pat. No. 5,528,248 describes how a GPS receiver can be integrated with a PDA to display navigation information for a user, it does not describe how positioning information provided to the PDA can be used in other ways.